H5N1-infected ducks may pose growing threat

Jul 19, 2005 (CIDRAP News) – A new study indicates that H5N1 avian influenza viruses are becoming less deadly to ducks, permitting them to carry the viruses for days or weeks and spread them to more susceptible birds and potentially to humans.

The findings "suggest that the duck has become the 'Trojan horse' of Asian H5N1 influenza viruses," says the report by an international team led by researchers from St. Jude Children's Research Hospital in Memphis. "The ducks that are unaffected by infection with these viruses continue to circulate these viruses, presenting a pandemic threat."

The researchers experimentally infected ducks with various H5N1 viruses, most of them dating to 2003 and 2004. About half of the infected ducks survived while shedding the virus for as long as 17 days, according to the report, published online today by the Proceedings of the National Academy of Sciences.

Wild waterfowl are the natural hosts for influenza A viruses (including H5N1) and commonly carry them without getting seriously ill. But the report says the biology of H5N1 viruses changed dramatically in late 2002, when they acquired the ability to kill many waterfowl species. Since late 2003, the viruses have spread widely in East and Southeast Asia, killing or forcing the slaughter of tens of millions of chickens and ducks and also infecting more than 100 humans.

The investigators, led by senior author Robert G. Webster of St. Jude's, obtained samples of 14 H5N1 viruses isolated from chickens, ducks, and humans in Vietnam, Hong Kong, Thailand, China, Indonesia, and Singapore. The majority of the samples dated from 2003 or 2004, but a few were from 1997 and 2001.

After growing the viruses in chicken eggs, the researchers inoculated each type into two mallard ducks. Two uninfected ducks were added to the cage with the infected ducks 4 hours after inoculation. The ducks were observed for 21 days, and tracheal and cloacal swabs were taken every other day and tested for the virus. Viruses isolated on day 17 were used to inoculate more ducks, and the procedure was repeated.

Six of the original set of 14 viruses were found to be highly pathogenic, meaning they killed at least one of the two ducks. All of the viruses were transmitted readily to the uninfected ducks.

In previous studies, ducks usually shed highly pathogenic H5N1 viruses in feces for only 2 to 5 days, but in this study the ducks shed the 2003 and 2004 strains for 11 to 17 days. In contrast, 1997 and 2001 versions of the virus were shed for no more than 7 days. A further observation was that the 2003 and 2004 viral strains were shed primarily from the upper respiratory tract rather than in feces.

To test whether viruses' pathogenicity changed during the course of infection, the investigators inoculated uninfected ducks with four viral strains shed by the first set of ducks after 13 and 17 days of infection. Though the original viruses were highly pathogenic, the day-13 and day-17 isolates caused no signs of illness in previously unexposed ducks. However, when chickens were exposed to day-17 isolates of two viruses dating to 2003 and 2004, all of them died.

"Our findings suggest a trend toward decreased pathogenicity of H5N1 viruses in ducks, although the small number of viruses and ducks tested (because of space constraints of the biosafety level 3+ facilities) precludes a firm conclusion," the report says. In view of other evidence that H5N1 viruses isolated from healthy ducks can sicken chickens and mammals, "the duck may be resuming its role as a reservoir of H5N1 viruses, transmitting them to other bird species and potentially to mammals."

The authors add that low-pathogenic H5N1 infection in ducks may be much more common in Asia than is currently suspected. They recommend surveillance of healthy poultry throughout Asia to determine if the virus is endemic in domestic birds and to clarify the role of domestic ducks in its spread.

"There is a real possibility that if these H5N1 viruses continue to circulate, further human infection will occur, increasing the potential for human-to-human transmission," the authors warn.